Triple-star system passed near Milky Way's central black hole

Sagittarius A swallowed one of them and ejected the other two out of our galaxy.

A hundred million years ago, a triple-star system traveled through the bustling center of our galaxy and made a life-changing misstep. The trio wandered too close to the galaxy's giant black hole, which captured one star and ejected the other two from the Milky Way.

This star, one of the fastest ever detected by 2010, is zooming through space at 1.6 million miles per hour (2.5 million kilometers per hour), three times faster than the Sun's orbital velocity in the Milky Way. Hubble's observations confirm that this stellar speedster originated from the Milky Way's core, resolving previous uncertainties about its origin.

In an unexpected twist, the two outbound stars merged to form a super-hot, blue star, according to a study published in The Astrophysical Journal Letters and illustrated on the NASA/ESA Hubble telescope website (pictured below).

This scenario, which might sound like science fiction, is the most likely explanation for the hypervelocity star HE 0437-5439, as per NASA's Hubble Space Telescope observations.

Since its discovery in 2005 by the Hamburg/European Southern Observatory sky survey, HE 0437-5439 has puzzled astronomers. Two theories were proposed: the star either rejuvenated by becoming a blue straggler, or it was ejected from the Large Magellanic Cloud, a neighboring galaxy.

In 2008, a team matched the star's chemical makeup to stars in the Large Magellanic Cloud, but Hubble's findings have settled the debate.

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Between 2005 and 2009, astronomers have discovered roughly 16 hypervelocity stars, most believed to be exiles from our galaxy's heart. However, this Hubble result is the first direct observation linking a hypervelocity star to the galactic center.

"Using Hubble, we can for the first time trace back to where the star comes from by measuring the star's direction of motion on the sky. Its motion points directly from the Milky Way center," says astronomer Warren Brown of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, a member of the Hubble team that observed the star. "These exiled stars are rare in the Milky Way's population of 100 billion stars. For every 100 million stars in the galaxy lurks one hypervelocity star."

Currently, this stellar outcast is located more than 200,000 light-years from the Milky Way's center, high above the galaxy's disk, which is approximately 100,000 light-years in diameter.

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Given that HE 0437-5439 travels at an extraordinary velocity, twice what it needs to escape the galaxy's gravitational pull, and no star are as fast under normal circumstances, there’s a strong suspicion that something unusual must have occurred, the researchers argued.

A perplexing aspect of is that, based on its speed and position, it must be 100 million years old to have journeyed from the Milky Way's core. Yet, its nine-solar-mass and blue color suggest it should have burned out after only 20 million years, far shorter than the time required to reach its current location.

The most plausible explanation is that the star was part of a triple-star system involved in a gravitational interaction with Sagittarius A, our galaxy's central black hole.

This theory, proposed in 1988, predicted that the Milky Way's black hole should eject a star about once every 100,000 years.

Blue stragglers are not uncommon in the Milky Way, and most stars are in multiple systems.

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